Error while running event multiple-characters::switch-to-character (ID 271)
LuaGuiElement API call when LuaGuiElement was invalid.
stack traceback:
[C]: in function '__index'
__multiple-characters__/gui.lua:38: in function 'build_gui'
__multiple-characters__/gui.lua:6: in function 'toggle_gui'
__multiple-characters__/control.lua:436: in function <__multiple-characters__/control.lua:426>

This happens when I open the colour customisation menu, then hit U to close it and open the Multiple Characters menu again. It happens every time.

edit
The issue seems to be that main_frame is nil when main_frame.add is called, likely because the main Multiple Characters window is closed when the colour customisation window is open. Best I can tell from a cursory glance.

Thanks for the report. This bug should be fixed now

Can confirm this is fixed. Opening the Multiple Characters menu while the colour selector is active now properly switches to the Multiple Characters menu. Thank you.